The present invention relates to a practical substrate, preferably flexible, for surface-enhanced Raman spectroscopy, and to an apparatus for using the substrate in trace analysis, particularly of organic compounds, in either a continuous or a static monitoring mode.
The U.S. government has rights in this invention pursuant to a contract awarded by the U.S. Department of Energy.
A number of optical spectroscopic techniques have been developed to characterize solid-gas (vacuum), solid-liquid (electrolyte) and solid-solid interfaces. In particular, the chemical identity of surface-adsorbed molecular species can be determined with specificity using surface analysis spectroscopy (SAS), such as infrared transmission spectroscopy and electron energy loss spectroscopy, instead of surface electronic absorption spectroscopy or photoacoustic spectroscopy. For example, SAS techniques can be used in the analysis of molecules sorbed at the surface of an electrode within a working electrochemical cell.
Among the SAS methods, surfce-enhanced Raman spectrometry (SERS) has recently received considerable attention. Enhancements by factors of 10.sup.3 to 10.sup.6 can be realized in the Raman scattering intensity for adsorbates on or near special rough metal surfaces. This phenomenon has been verified for adsorbates at silver, copper, and gold metal surfaces under both solution and vacuum conditions. See, e.g., Albrecht & Creighton, 99 J. AM. CHEM. SOC. 5215 (1977). These spectacular enhancement factors help overcome the normally low sensitivity of Raman spectroscopy which had often necessitated the use of powerful, costly laser sources for excitation.
In spite of the current interest in the SERS phenomenon, there has been no report on a generalized application of this effect for trace analysis. Most of the basic studies reported in the literature deal with samples of concentrations between 10.sup.-1 and 10.sup.-3 M, well above the concentration range of interest to analytical spectroscopists. Also, previous SERS studies have involved only rigid substrates and specific surfaces, such as glass plates covered with silver particles or the like and microscopically roughened electrodes, and have dealt mainly with highly polarizable, small monocyclic molecules, such as pyridine and its derivatives, and with a few ionic species, such as the cyanide radical CN.sup.- and the anion of dithiozone. See A. Otto in 6 APPLICATIONS OF SURFACE SCIENCE 309-55 (North-Holland Publ. Co. 1980); Pemberton & Buck, 53 ANAL. CHEM. 2284 (1981) Vo-Dinh et al, 56 ANAL. CHEM. 1667 (1984), and references cited therein. As a consequence, no information on the reproducibility and general applicability of the SERS technique is available.
Furthermore, one of the greatest barriers to the analytical applications of SERS, especially for continuous monitors, is the lack of practical substrate materials that can be easily prepared and that can provide data with sufficient reproducibility and accuracy for analytical purposes. Heretofore, rigid surfaces were prepared for SERS via a variety of techniques, such as electrochemical roughening of electrode surfaces, lithographic etching, and the "prolade post" method. In the prolade post method, a SiO.sub.2 support was first coated with a thin (4 to 5 nm) layer of etch-resistant metal, such as silver or aluminum, and the resulting metal layer was then disrupted by heating to form metal "islands" on the SiO.sub.2 surface. Thereafter, the substrate was exposed to an SiO.sub.2 -etching plasma, so that surface areas between the metal islands were etched to produce metal-capped "posts." After the metal caps were removed by a acid wash, a SERS-active metal was deposited, e.g., by thermal evaporation, onto the ends of the posts to produce the SERS substrate. This approach is elaborate and time-consuming.